1. Field of the Invention
The present invention relates to thermal processing equipment, and more particularly to a method and apparatus for achieving feed-forward temperature control in thermal processing equipment.
2. Description of the Related Art
Various apparatuses have been used to manufacture semiconductor devices, for depositing a thin film on a semiconductor wafer, forming an oxide film on the wafer and diffusing impurities into the wafer. Among them are a CVD (Chemical Vapor Deposition) apparatus, an oxide film forming apparatus and a diffusion apparatus. Recently, a vertical thermal processing furnace is used in these apparatuses, in order to perform high-precision process on semiconductor wafers. Most thermal processing furnaces of this type comprise a tubular furnace, a reactor and an elevator. The furnace extends vertically. The reactor is made of silica or the like, is located in the tubular furnace and defines a processing chamber. The elevator transports a wafer boat into and from the processing chamber. Many members to be processed, such as semiconductor wafers, are contained in the wafer boat. They are positioned horizontally and one above another. When the wafer boat is lifted into the reactor by the elevator, the members are brought into the chamber. A reactive gas is introduced into the chamber, whereby thermal process such as oxide film forming, is performed on the members.
To manufacture integrated circuits which operate at high speed and which has high integration density, the process at the surface of a semiconductor wafer must be controlled with high precision. To this end, it is necessary to increase the precision of thermal process. Particularly, the temperature needs to be increased and decreased uniformly in the processing chamber.
However, it is difficult to increase and decrease the temperature uniformly in the processing chamber. This is because the parts of the reactor has different thermal loss characteristics. Heat accumulates in the top of the reactor and leaks at the manifold connected to the bottom of the reactor. As a result, the temperature is higher in the upper part of the reactor than in the lower part thereof. Temperature cannot be distributed uniformly in the chamber even if the output of the heater provided outside the chamber is kept constant.
A method of uniformly distributing temperature in the processing chamber has recently been developed. In this method, a plurality of heaters are located in the heating zones of the chamber and independently controlled to equalize the temperatures in the heating zones.
However, even if the heaters are controlled at the same time, it remains difficult to increase or decrease the temperatures in the heading zones to the same value thereby to heating or cooling the members to be processed (e.g., semiconductor wafers) which are located in the heating zones. This is because the heating zones have different thermal transfer characteristics. In order to change the temperatures in the heating zones to the same value, a plurality of temperature controllers are used, each for one heater, different temperature control values (known as "ramping values") need to be set in the temperature controllers. Much time and skill are required to set different ramping values in the respective controllers, however. This is a practical problem. In the case where only one temperature controller is used, and different ramping values can not be set in the heating zones, respectively, it is impossible to uniformly distributing temperature in the processing chamber.
To accomplish successful thermal process on, for example, semiconductor wafers, it is necessary to heat the wafers from a low temperature (e.g., 500.degree. C.) to a predetermined temperature (e.g., 1000.degree. C.) within a short time, to maintain the wafers at the predetermined temperature (e.g., 1000.degree. C. ) during the thermal process, and to cool the wafers to the low temperature (e.g., 500.degree. C.) within a short time after the thermal process.
In the conventional vertical thermal processing furnace, the members to be processed are heated by the heater provided outside the chamber. Heat is transmitted from the heater is indirectly transmitted to the members. More precisely, the heat is transmitted to the members through the silica walls of the chamber or through means for uniformly transmitting heat. Some time elapses until the heat reaches the interior of the processing chamber. The temperature control effected by only the temperature detector located near the heater outside the chamber is inevitably delayed. Consequently, the temperature in the chamber cannot be controlled with accuracy and at high speed. This causes a problem. Once the temperature in the chamber has fallen due to the insertion of the wafer boat into the chamber, its recovery requires much time since the temperature control is delayed in the conventional method.
To shorten the temperature-recovery time, a temperature control system has been developed. The system has a temperature detector positioned in the processing chamber. Based on the temperature detected by the detector is applied to control the temperature in the chamber. In this system, however, a film is formed on the temperature detector during the thermal process, since the temperature detector is located within the chamber. Once covered with a film, the temperature detector can no longer accurately detect the temperature in the chamber, making it impossible to control the temperature with sufficient accuracy.
More recently, a temperature control method has been developed in which the temperature in the processing chamber is controlled based on only the temperature provided outside the chamber in accordance with the temperature control recipe prepared by interpolation on the basis of a look-up table. This method has a problem, however. Many parameters need to be input, and much skill is required, to prepare the temperature control recipe.